It is well known that plastic surfaces such as various polyolefins have poor adhesion with other materials. The modification of plastic surfaces by electrical discharge to improve adhesion or decoration has had wide commercial use.
The use of specially designed electrical discharge equipment for surface treating plastics for a myriad of adhesives has been met with great success in the container, medical, toy and automotive industries.
The modification of plastic surfaces by various methods to improve adhesion has been in existence in the converting and packaging industry for some time, and has been gaining considerable acceptance in other industries, such as the automotive industry. Although there has been extensive research done on treated polymer surfaces, a basic understanding of the mechanism by which surface treatment improves adhesion is limited. The reason for this lack of understanding is that the surface treatment process affects only the upper few hundred angstroms of a polymer surface and chemical analysis is difficult. However, surface treatment as used in this invention is generally defined as the preparation of a plastic surface by exposure to an electrical field to effect oxidization or other molecular changes in order to allow maximum adhesion between itself and other materials such as glue, inks, coatings and the like.
Application of an electrical field to provide such surface treatment is well shown in Leach & Williams U.S. Pat. No. 3,428,801 and Williams U.S. Pat. Nos. 3,715,306 and 3,722,661. Such treatment may be carried out in a conveyor tunnel for continuous treatment of a series of plastic objects such as Williams U.S. Pat. No. 3,722,661.
While such surface treatment has proved effective for the treatment of exterior surfaces, it has proven to be a problem to effectively treat the interior surfaces of hollow plastic objects particularly in a continuous system employing a conveyor tunnel for treatment.